Solid rocket motor

ABSTRACT

1. In combination with a vessel forming a rocket motor combustion chamber, a rubber-base liner having one surface firmly adhering to the chamber, the opposite surface of said rubber-base liner having a plurality of discrete powder grains partially embedded therein, each of said powder grains having a portion extending from said opposite surface of said rubber-base liner, and a propellant charge substantially filling the lined portion of said chamber, said propellant charge firmly adhering to said liner and enveloping each of said extending portions of said powder grains.

This invention relates to improvements in rocket motors and other gasgenerating devices which utilize solid propellants. More particularly,this invention relates to improvements in bonding characteristicsbetween the insulating liner of rocket motor combustion chamber wallsand the propellant charge contained within the chamber.

Rocket motors are generally comprised of a suitable vessel having anozzle in one end thereof, a combustion chamber containing a propellantcharge, and an ignition system capable of igniting the propellant chargewhen desired. Control of the thrust throughout the combustion period ofthe propellant may be obtained by shaping of the propellant to giveburning surfaces that will yield the desired thrust program during thecombustion period. The use of propellants in the form of a rod, rod withinternal star, hollow rod, multiple port rod, and the like, for thispurpose is well known in the art. In addition, certain desired surfacesof the propellant may be coated with a suitable inhibitor to suppressburning of these surfaces, thereby providing another means ofcontrolling the thrust. One method of inhibiting the burning surface ofthe propellant comprises coating the interior surfaces of the combustionchamber, after suitable surface preparation, with an insulating binder,and the exterior periphery of the propellant, by casting or otherwise isthen secured to the interior combustion chamber surfaces by means of theinsulating binder. The insulating binder may be used for severalpurposes. It may serve as an insulator for the rocket vessel to protectit against the high temperatures generated during combustion. It mayserve as a binder to secure the propellant to the wall of the combustionchamber. In addition, it may be used to inhibit burning of the outerperiphery of the propellant. Considerable difficulty has beenencountered in obtaining a satisfactory liner material that will serveall three purposes. Various elastomeric materials have been used as theinsulating liner, but the bond between the liner and propellant oftenfails if the rocket is stored for extended periods after fabrication orif the rocket is subjected to extreme variations in temperatureconditions prior to or during firing. Therefore it is necessary to firesuch rockets very soon after fabrication, and expensive and cumbersomemeans must be employed to maintain the temperature of the rocketrelatively constant prior to firing. Unless these precautions are taken,there may be failure of the bond between the liner and propellant, whichresults in uncontrolled burning of the propellant during the combustionperiod. Such uncontrolled burning not only causes undesired variationsin the thrust program, but also may cause rupture and/or explosion ofthe rocket vessel. In addition, when nitroglycerine is employed as acomponent of the propellant, there is a migration of the nitroglycerinethrough the propellant and liner to the chamber wall, which often causesfailure of the bond between the liner and wall, thereby resulting inuncontrolled burning of the propellant.

It is a primary object of this invention to overcome the disadvantagesinherent in conventional techniques for bonding rocket motor liners androcket motor propellant charges.

A further object of this invention is to provide an improved rocketmotor.

Still another object of this invention is to provide an insulating linerfor the combustion chamber of rocket motors having improved bondingproperties.

It is another object of this invention to provide an improved processfor preparing rocket motors.

Still a further object of this invention is to provide a rocket motorcombustion chamber having an insulating liner bonded to the propellantcharge, wherein the bond between the liner and the propellant chargeresists deterioration during extended periods of storage.

Another object of the invention is to provide a novel rocket motor linerwhich remains firmly bonded to the propellant when subjected to extremevariations in temperature.

It is a further object of the invention to provide a novel insulatingliner for rocket motors which inhibits migration of nitroglycerine frompropellants containing it.

These and other objects of the invention will be apparent from thefollowing detailed description thereof.

It has now been discovered that improved bonding between a rubbery linerof a rocket motor combustion chamber and a solid propellant charge isreadily obtained by coating the interior surfaces of the combustionchamber with a layer of a liquid organic material capable of being curedand solidified to a material having rubbery properties, and before thebinder has solidified, sprinkling solid powder particles on the exposedsurfaces of the binder, whereby the solid particles become partiallyembedded therein, and then curing and solidifying the rubbery linercomposition. The terms "partially embedded" and "partially embedding"are used throughout the description and claims to define the partialpenetration of the rubbery liner with a portion of each of the powderparticles, leaving an outer portion of each powder particle protrudingfrom the rubbery liner. After curing and solidification of the rubberyliner having the powder particles partially embedded therein, athermoplastic propellant charge is then cast into the lined portion ofthe chamber, and upon solidification of the propellant charge, astrongly adhering bond is formed between the rubbery liner, the powderparticles and the propellant charge.

FIG. 1 is a sectional elevational view of the novel rocket motor of thisinvention.

FIG. 2 is a sectional isometric view through plane 2--2 of FIG. 1, priorto the addition of the solid propellant.

Referring to FIG. 1, the rocket motor there shown consists of a shell 1constructed of steel, aluminum, or other suitable metal or material ofconstruction. Shell 1 has a cap 2 formed of the same or similar materialused to construct shell 1. Cap 2 encloses combustion chamber 3. A majorportion of combustion chamber 3 is filled with a charge comprised of aliner 4, powder particles 5 and solid propellant 6, each powder particle5 being partially embedded in liner 4 and partially embedded in solidpropellant 6. Ignition device 7, which is secured to cap 2, may be ofany design which will provide for ignition of solid propellant 6 at thedesired time. The ignition device 7 shown in the drawing is an electricsquib which when subjected to an electric current provides combustionproducts and/or flame which impinge on the exposed surface 8 of solidpropellant 6 to effect ignition thereof, and the resulting ignitionforms gaseous combustion products which are discharged through nozzleopening 9.

FIG. 2 is a sectional isometric view through plane 2--2 of FIG. 1 priorto the addition of solid propellant 6. Shown in FIG. 2 is shell 1 havingliner 4 with powder particles 5 partially embedded therein.

More in detail, rocket motor vessels, gas generator vessels, and thelike, constructed of a suitable material such as steel alloy, stainlesssteel, aluminum alloy, low carbon steel, ceramic lined steel, moldedepoxy resin-coated glass fiber, and the like, may be treated inaccordance with the technique of the instant invention. For purposes ofclarity, the invention will be described as applied to rocket motorvessels, but it will be recognized by those skilled in the art that theinvention is applicable to gas generators and other devices which employsolid propellants. The interior surfaces of the combustion chamber arecleaned by sanding and/or applying cleaning solvents to remove rust,grease and the like, to improve bonding between the interior combustionchamber surface and the liner.

The liner is formed from a liquid organic material capable of beingcured and solidified to a material having rubbery properties. It ispreferred to employ a mixture of an epoxy resin and a rubber-basematerial with conventional curing agents, since the epoxy resincomponent not only enhances the bonding strength of the liner but alsoimpedes the migration of nitroglycerine from the propellant to thecombustion chamber surfaces. However if the propellant does not containnitroglycerine, satisfactory results can be obtained using conventionalrubber-base materials.

The words "liquid" and "fluid" when used to define the organic materialcapable of being cured and solidified to a material having rubberproperties throughout the description and claims are intended to includesolutions, dispersions, slurries or paste-like fluids having theconsistency of conventional rubber-base cements.

Rubber-base materials suitable for use in preparing the liner of theinstant invention include fluid polymers or copolymers of one or morerubber-forming diolefins, such as butadiene and including substitutedbutadienes such as isoprene, chloroprene, dimethyl butadiene, methylpentadiene, and the like, with or without other suitable copolymerizablematerials such as acrylic acid, stryene vinyl esters, acrylic esters,methacrylic esters, and the like. Liquid dispersions or solutions ofnatural rubber, C rubber, nitrile rubber, GRS rubber, neoprene,polysulfide rubber, cellulose acetate, and the like, which may be curedand solidified by the application of a curing agent and/or heat may beemployed.

Curing agents for these rubber-base materials are well known in the art,and will vary with the type of rubber base material employed. Forexample, magnesium oxide, diphenylguanidine, p-quinonedioxime,peroxides, sulfur, and the like, are suitable curing agents.

The epoxy component of the rubbery liner of the present inventioncomprises liquid monomeric or polymeric polyepoxides containing aplurality of epoxide groups having the formula ##EQU1## At least one ofthese groups in the polyepoxides is terminal. The polyepoxides may besaturated or unsaturated aliphatic, cycloaliphatic, aromatic orheterocyclic and may carry non-interfering substituents, for example,chlorine, hydroxyl or ethyl groups. The polyepoxides are commonly formedby any of several reactions. One type is derived by the reaction ofaromatic or aliphatic polyhydroxy compounds with epichlorohydrin. Alsouseful in place of epichlorohydrin are 3-chloro-1,2-epoxybutane,3-bromo-1, 3-epoxyhexane, 3-chloro-1,2-epoxyoctane and the like. To formthe epoxy resins, the epoxide is reacted with an aliphatic or aromaticpolyhydric compound, for example, bisphenol A, resorcinol or polynuclearphenols, for example, 4,4'-dihydroxybenzophenone or bis(4-hydroxyphenyl)ethane. Sorbitol, glycerol and pentraerythritol are examples of suitablepolyhydroxy aliphatic compounds. Any of the commercially available epoxyresins are suitable for use in the present compositions including theresins sold under the names of Epon, Araldite, ERL and Epi-Rez. Epon 828is an epoxy resin formed from bisphenol A and epichlorohydrin and hasaverage molecular weights of 350 to 400 and viscosities of 5,000 to15,000 centipoises at 25°C. (Gardner Holdt). Epon 834 is similar to Epon828 but has molecular weights of about 450 and viscosities between A₁and A₂ (Gardner Holdt). Araldite 6010 is similar to Epon 828 but has aviscosity of 16,000 centipoises. ERL 2774 is similar to Epon 828 but hasviscosities of 10,500 to 19,500 centipoises. Epi-Rez 510 is similar toEpon 828 but has viscosities of 9,000 to 18,000 centipoises.

Other suitable epoxies include the Oxiron, peracetic and Novolac types.The Oxiron resins are epoxidized aliphatic polyolefins which containepoxy groups terminally and along the carbon chain. The peracetic resinsare obtained by epoxidation of olefins by oxygen and selected metalcatalysts or by peracetic acid. Unox Epoxide 201 is abicyclo-diepoxycarboxylate, Unox Epoxide 207 is bicylopentadiene dioxideand Unox Epoxide 206 is vinylcyclohexene dioxide. Novolac resin DER isan opoxidized polynuclear polyhydric phenol. Additional data concerningthese resins appears in Lee and Neville, "Epoxy Resins, TheirApplication and Technology," McGraw-Hill Book Company, Inc., New York,1957.

The uncured liquid organic material used in the preparation of the lineris prepared by admixing the rubber-base material with the epoxycomponent in a weight ratio of rubber-base material to epoxy componentof preferably between about 1:1 and about 5:1. As discussed above, theepoxy component may be omitted under certain circumstances. However,when the ratio is less than about 1:1, the high epoxy content mayeliminate most of the desirable elasticity properties from the liner.

Curing agents are added in a proportion up to about 20 per cent byweight of the liquid organic material and preferably between about 10and about 15 per cent by weight.

The liquid organic material capable of being cured and solidified to amaterial having rubbery properties, with or without an epoxy component,as the case may be, is applied to the interior surfaces of thecombustion chamber by any suitable means, such as by brushing, byapplying with a spatula, and the like. If desired the organic materialmay be formed or mixed with a suitable solvent such as ethyl acetate,acetate, or hexane, then the solution of organic material is sprayedonto the interior surfaces of the combustion chamber, and the solvent isevaporated to yield a rubbery liner. In one embodiment of the inventionthe cylindrical rocket motor vessel is placed on motor driven rollers orother suitable apparatus to rotate the rocket motor at a rate sufficientto develop the necessary centrifugal force to maintain a substantiallyuniform thickness of the layer of organic material solution on theinterior of the vessel until it is cured sufficiently to retain itslayer form without rotation. Application and rotation is continued untilthe desired thickness of rubber-base liner is obtained. The thickness ofthe rubber-base lining is preferably between about 1/4 and about 1/4inches, but any suitable thickness that will impart the desired degreeof insulation to the combustion chamber vessel may be employed.

If desired, several layers of rubber-base material and epoxy resin maybe employed to form the liner. For example, the combustion chamber wallis coated with a layer of the above defined liquid organic material, andafter curing, a thin layer of epoxy resin is applied. After curinganother layer of the liquid organic material is applied, which is coatedwith the powder particles, before curing, as defined below. Such acomposite liner is particularly suitable for inhibiting the migration ofnitroglycerin from propellants containing it.

Before complete solidification of the organic rubber-base material iseffected, solid powder grains are sprinkled on the exposed surface ofthe rubber-base lining. It is preferred to apply the solid powder grainsimmediately after applying the rubber-base liner to the combustionchamber vessel. However, the solid powder grains may be applied at anytime as long as the rubber-base liner is still tacky and is not yetsolidified. Generally a period of time up to about 10 minutes afterapplying the rubber-base liner to the combustion chamber is the maximumperiod in which satisfactory adhesion between the rubber-base liner andthe solid powder grains can be obtained. Materials suitable for use asthe powder grains include grains of nitrocellulose powder prepared byeither the well known "extrusion" process or the "Ball powder" process,cellulose acetate grains, methyl cellulose grains, ethyl cellulosegrains, benzyl cellulose grains, polystyrene grains, polyvinyl chloridegrains, polymethyl methacrylate grains, and mixtures thereof.

Powder grains having an average diameter between about 0.01 and about0.2 inches, and preferably between about 0.025 and about 0.05 inches areemployed. However, grains with an average diameter slightly smaller orslightly larger than these diameter ranges may be employed, the largergrains being employed when the thicker liners are used.

Sufficient powder grains are applied to the rubber-base liner to coatbetween about 50 per cent and about 95 per cent, and preferably betweenabout 65 and about 85 per cent of the exposed liner area. The grains aredistributed substantially uniformly over the entire liner area, withoutcompletely coating the liner.

The powder grains may be applied to the exposed surface of therubber-base liner by sprinkling by hand, by mechanical means, or by anyother suitable technique. In one embodiment the rocket motor vessel ispositioned with its longitudinal axis in a substantially horizontalposition, (the rocket motor vessel having the incompletely curedrubber-base liner applied to the combustion chamber), then rotated aboutits longitudinal axis, and powder grains are then forced through aperforated distributor pipe positioned parallel to the longitudinal axisof the rocket motor vessel on the interior of the combustion chamber,but removed from the rubber-base liner. However, any other suitablemeans of applying the solid powder grains to the exposed surface of therubber-base liner may be employed.

The rate of application of the solid powder grains is such thatsufficient powder grains are applied to obtain coverage of the surfaceof the rubber-base liner within the above defined ranges beforesolidification of the rubber-base liner is effected. The rate ofsolidification of the liner is determined by the proportion and type ofcuring agent employed and the temperature at which curing is effected.When the solid powder grains are applied to the rubber-base liner inthis manner, a major portion of the powder grains become partiallyembedded in the rubber-base liner and protrude from the surface of theliner. A small portion of the powder grains will become completelyembedded in the rubber-base liner and a negligible portion of the powdergrains may not adhere to the surface of the rubber base liner. Thesenon-adhering powder grains are preferably removed from the combustionchamber prior to adding a propellant charge as described more fullybelow.

After applying the powder grains to the liner in this manner,solidification of the rubber-base liner is completed, due to the effectof the curing agent, with or without heating. It is preferred to employa curing agent and complete curing of the liner at ambient temperaturefor a period at least about 12 hours and as long as about 10 days ormore. However, solidification of the liner may be effected at elevatedtemperatures, for example, up to about 80°C., in less than about 6hours.

Rocket motor casings lined and powdered in accordance with the techniqueof the instant invention may be stored for extended periods, forexample, as long as 90 days before adding the propellant charge, withoutadversely affecting the bonding characteristics of the liner to thepropellant.

Any thermoplastic propellant charge may be employed that is capable ofbeing poured and cast inside of the combustion chamber of the rocketmotor and subsequently solidified by curing. Suitable propellant chargesinclude the rubber-base and asphalt-base propellants containingoxidizers such as ammonium perchlorate, potassium perchlorate, ammoniumnitrate, and the like; propellants such as nitrocellulose,nitroglycerine, cyclonite, pentaerythrite tetranitrate and mixturesthereof; and metal additives such as powdered aluminum, boron, copperand mixtures thereof. A typical analysis of a suitable thermoplasticpropellant composition is as follows:

    Component         Parts by Weight                                             ______________________________________                                        Nitrocellulose    58.6                                                        Nitroglycerin     24.2                                                        Dimethyl Phthalate                                                                              9.6                                                         Dinitrotoluene    6.6                                                         Ethyl Centralite  1.0                                                         Carbon Black      0.1                                                         ______________________________________                                    

Other suitable solid propellant compositions such as double basepropellants, composite double base propellant and those set forth inRocket Propulsion Elements, by George P. Sutton, published by John Wileyand Sons, Inc., Second Edition (1956), may be employed.

The shape of the propellant may be in the form of a rod, hollow rod,star, etc., or other desired shape that can be cast in accordance withprior art techniques.

After solidification of the propellant charge a suitable closure meanshaving a nozzle for discharging combustion gases is then secured to theopen end of the chamber. A suitable ignition means is also provided inthe conventional manner.

When solid propellant grains are applied to the interface between arubber-base liner and a castable propellant charge in accordance withthe instant invention, an exceptionally strong bond is obtained betweenthe rubber-base liner and the propellant charge, and substantially norupturing of the bond is effected during storage, during extremetemperature variations, or when the propellant charge is ignited. As aresult, substantially uniform burning and uniform thrust are obtainedthroughout the entire combustion period. In addition, it has been foundthat rocket motors prepared in this manner can be stored indefinitelywithout any significant deterioration in the bond between therubber-base liner and the propellant charge. Furthermore, when subjectedto extreme temperature variations, for example, cycling tests in whichthe rocket motors are stored at a temperature of -40°C. for 24 hours,then removed and immediately placed in an atmosphere at 60°C., andmaintained in this atmosphere for 24 hours, and then returned to storageat -40°C., the rocket motors of the instant invention resisted bondfailure after being subject to more than 20 cycles.

In a preferred embodiment of the invention the powdering technique ofthe instant invention is applied to a liner of the type described in mycopending application, Ser. No. 143,274, filed Oct. 2, 1961. The liquidorganic material containing epoxy resin, rubber-base material, andcuring agent, prepared as defined above is admixed with an inorganicmetal salt such as zinc chromate, the proportion of salt beingequivalent to between about 2.5 and about 30 per cent by weight of theliquid organic material. The resulting slurry is then applied to thecombustion chamber wall as described above.

The following examples are presented to illustrate the invention morefully without any intention of being limited thereby. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE I

A rubber-base lining composition was prepared as follows: 35 parts of anepoxy resin having a molecular weight of about 375 formed by reactingepichlorohydrin with bisphenol A, were admixed with 5 parts of powderediron oxide until substantially homogeneous. To this mixture were added52.3 parts of a copolymer of butadiene and acrylic acid, and mixing wascontinued until a substantially homogeneous mass was formed. A curingcomposition was prepared by admixing 5 parts of p-quinonedioxime, 0.2parts diphenylguanidine and 2.5 parts magnesium oxide, and the resultinghomogeneous curing composition was then admixed with theresin-rubber-base-iron oxide mixture until substantially homogeneous.All mixing was carried out at room temperature.

A tensile testing apparatus was constructed which was comprised of twosquare steel plates, having a surface area on each face of 2.5 in.²,each plate being secured on one face to a pivot arm, which wasoperatively connected to a tensile strength measuring device. Theunattached faces of the two plates are cemented together to form a"sandwich" of steel plate/rubber-base liner/thermoplasticpropellant/rubber-base liner/steel plate. The pivot arms then force thesteel plates apart, the force necessary to cause rupture of the sandwichbeing measured in pounds per square inch.

A portion of the fluid rubber-base composition, prepared as describedabove, was applied to the open face of each steel plate with a spatulato form a layer of substantially uniform thickness (about 1/32 inch ).nitrocellulose powder grains having an average diameter of about 0.0315inches were then immediately sprinkled on the exposed surface of therubber-base liner on each plate in an amount sufficient to cover about75 percent of the exposed area. The nitrocellulose powder grainscontained 88 percent nitrocellulose (12.6 percent N, (5 percent leadstearate, 2 percent 2-nitrodiphenylamine, and 5 percentdioctylphthalate.

The rubber-base liner, having nitrocellulose grains partially embeddedtherein, was cured at ambient temperature for 2 days. The two plateswith rubber-base liner and powder grains were then cemented togetherwith a 1/2 inch thickness of a thermoplastic composite double basepropellant containing nitroglycerin, nitrocellulose, ammoniumperchlorate, powdered aluminum and additives. The cementing was effectedby placing the two lined plates vertical in a parallel position about1/2 inch apart, then taping the bottom and two adjacent sides with adouble thickness of masking tape to form a mold. The fluid propellantwas then poured into the top of the mold to fill it, and the tapedplates with propellant were heated to a temperature of 60°C. for about24 hours to effect curing of the propellant. The sandwiches were thencooled to ambient temperature and the tape was removed.

After curing of the "sandwich", the plates were pulled apart in thetensile strength testing apparatus. The maximum tensile strength andposition of the break were noted. The procedure was repeated for twoadditional sandwiches prepared in the same manner. The average tensilestrength of the three specimens was 86 pounds per square inch, and ineach test, the break in the sandwich occurred through the propellant,thus demonstrating that the bond formed by the technique of the instantinvention between the liner and propellant was stronger than the bond ofthe propellant alone.

For purposes of comparison, the procedure was repeated, with theexception that no powder grains were added to the liner. The averagetensile strength of the three specimens was only 66 pounds per squareinch, and in each case the break occurred at the liner-propellantinterface.

EXAMPLE II

A liner composition was prepared as in Example I, employing thefollowing components in the following proportions:

    Component              Parts                                                  ______________________________________                                        Epoxy Resin (of Ex. I) 32.5                                                   Zinc chromate          7.5                                                    Butadiene-acrylic                                                              acid copolymer        52.3                                                   p-quinonedioxime       5.0                                                    diphenylguanidine      0.2                                                    magnesium oxide        2.5                                                    ______________________________________                                    

Three sandwich specimens were prepared as in Example I, employing thesame type of powder grains and the same type of propellant. The averagetensile strength was 96 pounds per square inch, and the break occurredthrough the propellant in each instance.

EXAMPLE III

A rubber-base material prepared in the same manner as in Example II wasapplied to the interior surface of the combustion chamber of a 3 poundrocket motor which was comprised of a steel pipe of about 5 inchesinside diameter and about 3 1/2 inches in length, after sanding andsolvent cleaning of the interior surface. A ring was secured at each endof the pipe, the opening in each ring having a diameter of 1/4 inchesless than the pipe, and the resulting assembly was placed in a lathechuck and rotated at the rate of about 120 rpm. The liquid rubber-basematerial was then fed to the interior of the vessel, a spatula wasplaced in contact with both rings to spread the material in asubstantially even layer of about 1/8 inch thickness and to removeexcess material by scraping. Nitrocellulose powder grains of the typeused in Example I were sprinkled on the liner immediately to cover about75 percent of the exposed area. Rotation was stopped aftersolidification of the liner was finished and the liner was cured in therocket motor at ambient temperature for 51 days. The rings were removed,the edges of the liner squared, and the rocket motor was filled with thethermoplastic propellant of Example I. A star mandral was placed alongthe axis of the motor to form a star propellant. After curing thepropellant, the motor was cycled between -40°C. and 60°C. for 24 hourperiods for more than 18 cycles without failure of the liner-propellantbond. Several test motors prepared in this manner were firedsuccessfully after 3 to 6 temperature cycles.

EXAMPLE IV

A 70 pound JATO rocket motor was prepared, employing a rubber-basematerial similar to that of Example III, with the exception that therubber-based liner contained 5.0 parts of zinc chromate and 35 parts ofthe epoxy resin. The rocket motor casing was placed on rollers androtated at a rate of about 60 rpm. The paste-like rubber-base materialwas applied with a spatula to the interior surfaces of the combustionchamber to yield, after curing and solidification, a rubbery liner ofabout 1/8 inch in thickness. The propellant of Example I was then castin the rocket and cured. The completed rocket motor was subjected to twocycles at temperatures between -40°C. and 60°C. for 24 hour periodswithout failure of the bond between the liner and metal or liner andpropellant.

EXAMPLE V

A liner composition was prepared as in Example I, employing thefollowing components in the following proportions:

    Component              Parts                                                  ______________________________________                                        Epoxy resin of Ex. I   32.5                                                   Zinc chromate          7.5                                                    Butadiene-acrylic                                                              acid copolymer        52.3                                                   p-quinonedioxime       5.0                                                    diphenylquanidine      0.2                                                    magnesium oxide        2.5                                                    ______________________________________                                    

Three sandwich specimens were prepared as in Example I, except thatcellulose acetate particles were used instead of nitrocellulose grains,and the rubber-base liner was cured for four days instead of 2 days. Theaverage tensile strength was 103 pounds per square inch, and the breakoccurred through the propellant in each instance.

EXAMPLE VI

A procedure similar to Example I was employed with the exception that apolysulfide type rubber was employed as a liner in the preparation ofthree sandwich specimens, employing the powder grains of Example I. Theaverage tensile strength was 49 psi., and substantially all of the breakwas through the propellant. For purposes of comparison, employing thepolysulfide type rubber without any powder being sprinkled on the liner,an average tensile strength of only 15 psi. was obtained and the breakoccurred at the liner-propellant interface.

It will be recognized that many modification and variations, some ofwhich are discussed above, will naturally present themselves to thoseskilled in the art without departing from the spirit of this inventionor the scope of the appended claims.

Having thus described the invention,

What is claimed is:
 1. In combination with a vessel forming a rocketmotor combustion chamber, a rubber-base liner having one surface firmlyadhering to the chamber, the opposite surface of said rubber-base linerhaving a plurality of discrete powder grains partially embedded therein,each of said powder grains having a portion extending from said oppositesurface of said rubber-base liner, and a propellant charge substantiallyfilling the lined portion of said chamber, said propellant charge firmlyadhering to said liner and enveloping each of said extending portions ofsaid powder grains.
 2. The combination of claim 1 wherein saidrubber-base liner is the reaction product of an epoxy resin and acopolymer of butadiene and acrylic acid.
 3. The combination of claim 1wherein said rubber-base liner is the reaction product of an epoxy resinand natural rubber.
 4. The combination of claim 1 wherein said powdergrains are nitrocellulose powder grains.
 5. The combination of claim 1wherein the powder grains substantially uniformly cover between about 50and about 95 percent of the area of said liner.
 6. The combination ofclaim 1 wherein said rubber-base liner contains between about 2.5 andabout 30 percent by weight of an inorganic metal salt.
 7. Thecomposition of claim 6 wherein said metal salt is zinc chromate.
 8. Thecombination of claim 1 wherein the said powder grains have a diameterbetween about 0.01 and about 0.2 inches.
 9. The combination of claim 1wherein the powder grains have a diameter between about 0.01 and about0.2 inches and the powder grains substantially uniformly cover betweenabout 50 and about 95 per cent of said opposite surface of said liner.10. The combination of claim 1 wherein said rubber-base liner is thereaction product of an epoxy resin and a copolymer of butadiene andacrylic acid, said reaction product containing between about 2.5 andabout 30 percent by weight of zinc chromate, and wherein said powdergrains are nitrocellulose powder grains having an average diameterbetween about 0.01 and about 0.20 inches.